1. Field of the Invention
The present invention relates to an improved method for providing corrosion resistance to ferrous metal surfaces. More particularly, this invention relates to the application of zinc-rich primers to ferrous metal substrates in a new and improved manner so as to result in coated ferrous metal substrates with excellent physical properties including corrosion resistance and adhesion of the zinc-rich coating to the substrate particularly under a shearing force.
2. Brief Description of the Prior Art
The use of zinc-rich primers is becoming increasingly important in the automotive industry. The general thinking in the automotive industry is that cars and trucks must be built to last much longer. To do this, it is necessary that the steel bodies of the cars and trucks be made increasingly corrosion resistant. Thus, topcoats with better sealing properties, such as electrodeposited topcoats, and primers which provide better adhesion and corrosion resistance are becoming increasingly more important. Zinc-rich primers, because of their ability to offer galvanic protection and their electroconductivity enabling them to accept electrical resistance welding and subsequent electrodeposited topcoats, offer great potential to the automotive industry.
An example of the prior art dealing with zinc-rich primers is U.S. Pat. No. 3,671,331 to Malkin et al. The coating systems described in this patent are quite complicated and involve a first pretreatment of the ferrous metal substrate with a mixture of hexavalent chromium such as chromic acid and zinc dust which is then sintered at a relatively high temperature of 475.degree.-500.degree. F. (246.degree.-260.degree. C.) to bond the zinc and chromium to the metal surface. Over this pretreated surface is then applied a zinc-rich primer which consists of about 60 to 80 percent by weight zinc dust, 10 to 30 percent by weight of an organic binder and 10 to 30 percent by weight of a suitable diluent. The primer-coated substrate is then baked to harden the primer and remove volatiles. Ferrous metal substrates coated in this way are electroconductive, enabling them to be welded and subsequently electrocoated.
There are a number of disadvantages associated with the process described in U.S. Pat. No. 3,671,331. First of all, it is a two-step process requiring a complicated first step. The ferrous metal must be pretreated with a mixture of hexavalent chromium and zinc dust and this mixture sintered onto the substrate. The sintering requires high temperature and significant energy consumption.
U.S. Pat. No. 3,671,331 specifically mentions that the first step is absolutely necessary to provide good properties and corrosion protection; the implication being that the application of the zinc-rich primer directly to the metal substrate would not be effective. In fact, the reference cautions against the use of resinous materials in the first step, saying that the presence of more than very minor amounts of resin can deleteriously affect coating integrity with respect to weldability and galvanic protection.
Somewhat surprisingly, it has been found that zinc-rich primers can be applied simply and directly to ferrous metal substrates without the need to pretreat the substrate with hexavalent chromium-zinc dust composition disclosed in the prior art. It has been found that if the ferrous metal substrate is first etched with nitric acid and the zinc-rich primer applied directly to the previously etched surface, the primed surface has outstanding properties. The adhesion, ductility, flexibility and corrosion resistance of the coated substrate are as good or better than that of the prior art.